1. Field of Invention
The present invention relates to a power supply, and particularly to a voltage-regulator and a power supply having a current-sharing control.
2. Description of the Related Art
Voltage-regulators are commonly used in the power management systems of PC motherboards, notebook computers, mobile phones, and many other products. Power management systems use voltage-regulators as local power supplies, where a stable output voltage and a fast transient response are required. Voltage-regulators enable power management systems to supply additional voltage levels that are lower than the primary supply voltage. For example, the 5V power systems of many PC motherboards use voltage-regulators to supply local chipsets with a stable 3.3V voltage.
In spite of poor power converting efficiency, voltage-regulators generally have advantages of low cost, smaller size and little frequency interference. Particularly, voltage-regulators can provide a local circuit with a stable voltage that is unaffected by current fluctuations from other areas of the power system. Voltage-regulators are widely used to power local circuits when the power consumption of the local circuit is negligible with respect to the overall load of a power system.
FIG. 1 shows a typical circuit of a conventional voltage-regulator. Referring to FIG. 1, a voltage-regulator 5 comprises an input terminal IN for receiving an unregulated DC input voltage VIN, a pass transistor 10, an output terminal OUT for outputting a regulated DC output voltage VO and a voltage divider having resistors 31 and 32. The voltage-regulator 5 further comprises a feedback control circuit coupled to the pass transistor 10. The feedback control circuit comprising an error amplifier 20 is connected to the output terminal OUT of the voltage-regulator 5 via the voltage divider. The resistors 31 and 32 are connected in series from the output terminal OUT to a ground terminal GND of the voltage-regulator 5. A voltage-dividing node between the resistor 31 and the resistor 32 is connected to a positive terminal of the error amplifier 20. A reference voltage VREF generated by a band-gap unit 40 is supplied to a negative terminal of the error amplifier 20. An output terminal of the error amplifier 20 generates a gate voltage to a gate of the pass transistor 10. The feedback control circuit regulates the gate voltage for the pass transistor 10 to control the impedance thereof. In response to the gate voltage, the pass transistor 10 supplies the output terminal of the voltage-regulator 5 with various current levels. In this manner, the modulated gate voltages enable the voltage-regulator to output a stable DC voltage regardless of load conditions and input voltage variations.
The voltage-regulator 5 has an enabling terminal EN to enable or disable the voltage-regulator 5 for power management. For example, when a voltage at the enabling terminal EN is lower than a threshold voltage, the voltage-regulator 5 will be disabled. A transistor 11, acts as a switch, is coupled to the enabling terminal EN. Under normal operations, the voltage at the enabling terminal EN is pulled up by a resistor 36 at a high level, namely in an enabled status. When the voltage at the enabling terminal EN is lower than the threshold voltage, the transistor 11 is cut off. Consequently, as the transistor 11 is cut off, through a resistor 35, transistors 12 and 13, and NOT gates 25 and 26, the pass transistor 10 and the error amplifier 20 will be turned off. As the voltage-regulator 5 is disabled, only little quiescent current is consumed for saving power. Thus, the enabling terminal EN is a valuable and necessary interface to enable the voltage-regulator 5 meeting the power management requirement.
One drawback of conventional voltage-regulators is high operation temperature, especially as the input voltage is high. Another drawback is that an output current IO and a voltage drop VD of the pass transistor 10 will produce a power consumption PD, which increases an operating temperature of the voltage-regulator 5. Since the lifespan of the voltage-regulator 5 is closely related to the operating temperature thereof, in order to improve the reliability, the operating temperature must be reduced. The operating temperature of the voltage-regulator 5 largely depends on the packaging thereof. The packaging determines a thermal resistance and confines a heat radiation thereof. However, a lower thermal resistance of the packaging increases the manufacturing cost.